Sulfur is nearly ubiquitous in fossil fuels and occurs as inorganic (pyritic) sulfur and organic sulfur (mercaptans, disulfides, thiols, sulfones, thioethers, thiophenes) and many more complex forms of “bound” sulfur. In petroleum, it is the third most abundant element after carbon and hydrogen, and yet it is an undesirable component of both raw and refined fuels. The sulfur concentration of petroleum has been correlated with the corrosion of pipelines, pumps, and refining equipment and leads to the premature breakdown of engines. In addition, combustion of sulfur-containing fuels results in sulfur dioxide pollution of the atmosphere, contributing to acid rain. Consequently, strict regulations on sulfur emissions and the sulfur content of refined fuels have been adopted in the U.S. and elsewhere.
When the sulfur is predominantly in the organic form it can be removed chemically by a hydrodesulfurization process which involves reacting the hydrocarbon with hydrogen gas in the presence of a catalyst at elevated temperatures. Since the hydrodesulfurization process has many shortcomings and is quite expensive, microbial desulfurization (MDS) processes have attracted much attention.
Many processes utilizing microorganisms known to be capable of both degrading sulfur compounds and utilizing hydrocarbon for growth have been studied with varying degrees of success. However to date, Microbial Desulfurization has been shown to be effective only in laboratory experiments. Further, it has been suggested that current Microbial Desulfurization processes are, for the most part, merely incidental to the metabolic consumption of the hydrocarbon by the microorganisms during their growth process (catabolic MDS) rather than sulfur specific or sulfur selective reactions.
Sulfur content of carbonaceous fuels, such as coals and oils, has prevented utilization of a considerable amount of such materials due to deleterious effect upon the environment. Inorganic pyritic sulfur and organically bound sulfur may each constitute as much as about 3.5 weight percent of the coal. Microbial metabolism of inorganic pyritic sulfur by its oxidation using bacteria such as Thiobacillus and Sulfolobus species is known (Eligwe, C. A., “Microbial Desulfurization of Coal,” Fuel, 67: 451–458 (1988)). These chemolithotropic organisms utilize inorganic pyritic sulfur compounds as energy sources and are capable of removing 90% or more of the inorganic pyritic sulfur from coal within a few days.
Pre-combustion MDS for coal and liquid petroleum products has been described in, for example, U.S. Pat. No. 4,861,723 pyrite removal during wet grinding of coal with MDS organisms; U.S. Pat. No. 4,851,350 organic sulfur removal in coal slurry with added nutrients and Hansenula sp. or Cryptococcus albidus; and U.S. Pat. No. 5,510,265 for removal of organic sulfur by Rhodococcus species and their enzyme derivatives in combination with hydrodesulfurization (HDS). MDS in these cases is specific to removal of either pyritic or organic sulfur but not both during active processing of the fuel sources.
Valentine in U.S. Pat. No. 5,593,889 suggests use of MDS at any time during storage, transport, and processing of hydrocarbon fuels. However, the MDS process in '889 converts organic sulfur to water-soluble sulfates in an emulsion under growth conditions with supplemental nutrients.
Dibenzothiophene (DBT) is the organosulfur compound most considered representative of the form in which organic sulfur exists in naturally occurring organic carbonaceous fuels such as coal and oil and is the primary compound upon which the microbial metabolism of organosulfur compounds has focused. The pathway of microbial degradation of DBT in most of the prior art is by C—C bond cleavage. Microbial degradation of organic sulfur-containing carbonaceous materials by C—C bond cleavage results in the loss of a large portion of the calorific value of the carbonaceous fuel. It is, therefore, desirable to follow a microbial degradation route which removes sulfur from the molecule without removing carbon from the molecule, thereby retaining calorific value of the fuel to a greater degree than is possible by carbon degradative pathways. Such sulfur-specific metabolism of the organic substrates requires cleavage of carbon-sulfur bonds in the organic sulfur-containing molecule. In the case of sulfur specific metabolism of dibenzothiophene, the organic end product is 2-hydroxybiphenyl.
Prior art microorganisms alleged to be capable of degradation of DBT by C—S cleavage to sulfates include a Pseudomonas species as described by Isbister, et al. (Isbister, J. D. and Kobylinski, E. A., “Microbial Desulfurization of Coal in Processing and Utilization of High Sulfur Coals,” Coal Science and Technology Series, No. 9, 627); Rhodococcus rhodochrous and Bacillus sphaericus as disclosed by Kilbane, II, in U.S. Pat. No. 5,358,869; and Pseudomonas ATCC 39381 as set forth by Isbister, J. D., and R. C. Doyle in U.S. Pat. No. 4,562,156. However these organisms are intended for organic, not pyritic, sulfur removal under growth nutrient supplemented conditions and/or agitated aqueous emulsion conditions.
Johnson et al. in U.S. Pat. No. 6,071,738 disclose the use of recombinant microorganisms to remove organic sulfur by metabolic processing to organosulfinate and/or organosulfonate precipitates. These compounds are then extracted by a polar solvent and removed by phase separation using a polar phase such as water. Microorganisms or enzymes employed as biocatalysts in '738 are reported not to consume the hydrocarbon framework of the former refractory organosulfur compound as a carbon source for growth. As a result, the fuel value of substrate fossil fuels thus treated does not deteriorate. The concentrations of microorganisms or derived enzyme biocatalyst can be adjusted so that appropriate volumes of biocatalyst preparations having predetermined activities can be obtained. However these organisms are intended only for organic, not pyritic, sulfur removal. Further, use is described as under bioreactor or refinery type conditions.
Similarly Olson in U.S. Pat. No. 6,124,130 describes microbial desulfurization of hydrocarbon fuel without the fuel being used as a carbon or energy source. However at least a 500-fold growth of the culture is expected per week using a sulfur free added alternative carbon source in a supplemented innoculum.
U.S. Pat. No. 5,804,435 describes a Pseudomonas putida that is resistant to organic solvents and hence can be used for biodesulfurization with a very small amount of water. These microorganisms are organic solvent-resistant strains capable of performing desulfurization under microaerobic conditions in the presence of organic solvents. The application of these microorganisms to petroleum refining or coal desulfurization steps is suggested to establish a more efficient, energy-saving and safe desulfurization process. However no specific method of use is suggested.
Jenneman et al. in U.S. Pat. No. 5,789,236 describe the use of concentrated Campylobacter sp. on oil to speed up a reduction of sulfides process in fuel reservoirs. One example uses a microbe innoculum concentration of 107/ml. of nutrient supplemented culture medium.
Hence it is clear that none of the prior MDS art describes a microbiological method of desulfurization of fossil fuels (MDS) allowing the biocatalytic removal of sulfur without an unacceptable loss in the fossil fuel value due to catabolic destruction of the hydrocarbon, without the need for agitation, and without the addition of growth enabling nutrients. Further it is clear that none of the prior MDS art describes a microbiological method of desulfurization of fossil fuels (MDS) allowing the biocatalytic removal of both inorganic and organic sulfur without an unacceptable loss in the fossil fuel value due to catabolic destruction of the hydrocarbon and without the addition of growth enabling nutrients. The present invention therefore provides a new means for more effective and desirable MDS treatment of sulfur-containing hydrocarbon fuels.